BF: supposed fix for the gh-439, but still unable to reproduce OP.
This doesn't do it either: we need somewhere were start_params is OK, but then
RESTORE fails.
RF + TST: Removed some (unused? Definitely untested!) cruft. More test
coverage.
TST: Test that a sequence of sigmas also works.
BF: Set min_signal to 0.0001 so that we can deal with normalized signal.

See https://github.com/nipy/dipy/issues/451
RF: Better error handling and better handling of all-zero signals.
Whitespace.
Merge pull request #447 from arokem/restore-errors

BF: supposed fix for the gh-439, but still unable to reproduce OP.